rbinstall.rb: add date
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42890 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
rbinstall.rb: add date
RUBY_RELEASE_DATE. [ruby-core:57072] [Bug #8878]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42890 b2dd03c8-39d4-4d8f-98ff-823fe69b080e